Add experimental context management activation

[copyberry]

Add experimental context management activation

What changed

• Add the under-development features.context_management.experimental_mode configuration and schema.
• For eligible ChatGPT Plus, Pro, and Pro Lite sessions using the Codex backend, enable token-budget context, history notes, and the new_context tool when experimental context management is configured.
• Keep the feature disabled for custom providers, provider credentials, non-Codex endpoints, and temporary structured threads.

Testing

• Cover subscription eligibility, backend restrictions, configuration resolution, token-budget guidance, and temporary-thread overrides.

[github-actions]

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